Most traditional absorbent structures consist of a static network of fibers which contain a plurality of open areas located between the fibers. The open areas retain aqueous fluid which is absorbed by the absorbent structure. The majority of fluid is not absorbed into each individual fiber but instead most fluid is retained within the empty spaces which are formed in the network of cellulosic fibers. If the traditional absorbent member has a high absorbent capacity it usually does not have a high wicking rate. The reason for this is that the first attribute is in conflict with the second attribute.
Efforts to find absorbent members which have both a high absorbent capacity as well as a high wicking rate have only been marginally successful. It has been recognized that the dynamic properties of the fibers themselves somehow have to be changed. Some success has been obtained in calendering a wet laid network of bleached chemi-thermo-mechanical pulp (BCTMP). For this material, small expansion or release of potential energy upon wetting of the absorbent fibers was observed which can enhance the absorbent capacity and wicking rate of the absorbent member. It is believed that this occurs because the absorbent fibers are oriented, to a large extent, in the horizontal plane but with some modest "z" direction to the fiber axis as they conform to an irregular surface of the forming wire. The high to low position of the forming wires is about 0.020 to about 0.025 inches (about 0.508 mm to about 0.635 mm). When the tissue sheet is hot calendered at high pressure, this high to low shaping (or bumps in the sheet) is smoothed out. It is believed that the heat mobilizes any water present in the fibers, and the close proximity of fiber surfaces (intra and inter) allows hydrogen bonds to form with very little water present. When the flattened sheet is exposed to water, the hydrogen bonds break and the fibers return to their original shape. Since the wet laid sheet has a wet strength agent added, e.g., Kymene, the fibers stay attached to each other in the network therefore the sheet returns to its original bumpy state before calendering. These bumps or pockets on the surfaces hold more moisture than the flat sheet but a large portion of that moisture is not bound within the sheet structure.
It has also been observed that the open spaces within the fiber structure (void volume) of most traditional absorbent members, such as a paper towel, is limited. This is primarily due to two characteristics. First, the traditional absorbent member is restricted from expanding by the presence of wet strength bonds which limit or reduce the ability of the absorbent member to expand. Second, the axis of fibers of a traditional absorbent member are essentially oriented in only the x and y directions, not in the z-direction. This limits the absorbent member from being able to expand in three directions, thus reducing both its absorbent capacity and wicking rate.
Other attempts to increase the absorbent capacity and wicking rates of a traditional absorbent member have included the addition of superabsorbent particles (SAP). Superabsorbent particles have the ability to expand in size as they absorb fluid and also have the ability to retain fluid. However, the use of superabsorbent particles is disadvantageous in that most are very expensive and some of them tend to be rather slow in absorbing fluid. In addition, the relative absorbent capacity of most superabsorbent material is adversely affected by pressure and by ionic salts which are present in certain aqueous fluids, such as urine. Therefore, they present certain drawbacks to being used in disposable absorbent products such as diaper, training pants, incontinence garments, feminine napkins, meat and poultry pads, and the like.
Now it has been recognized that there is a real need for an absorbent member which has both a high absorbent capacity and a high wicking rate as well as the ability to rapidly expand in at least one direction when wetted by an aqueous fluid.